Natural gas at natural-gas wells or the wellheads of petroleum recovery installations frequently contains, as will be discussed in greater detail, below, components which can be removed from the natural gas and which may be valuable in their own right, for example, higher molecules weight hydrocarbons. Furthermore, the natural gas is at an elevated pressure at such sources, the latter term being used to refer to any location at which natural gas is capable of undergoing a dewpoint separation of selected components, available at an elevated pressure.
The term "dewpoint separation" as used herein will be understood to refer to a system (method or apparatus) wherein a gas mixture is cooled to a temperature below the dewpoint of a component to be recovered, so that this component condenses from the gas and is thereby separated out.
Mention may also be made herein to a "consumer" for the natural gas fraction from which condensible components have been removed by dewpoint separation and this expression will be used to mean any system which is capable of utilizing natural gas or transporting the same away from the dew point separator. For example, the consumer may be a pipeline or a distribution network for natural gas or methane, a liquefaction plant producing liquid methane, a transport pipeline for carrying the remaining natural gas long distances, or the like.
It is already known to cool natural gas, which appear at an elevated pressure at the source, by expanding it in an expansion turbine and thereafter separating out the condensible components, prior to pressurizing the balance, i.e. the "lean" or methane portion, and feeding the compressed gas portion to a pipeline.
The expansion turbine here functions, as in any expansion system, as a cooling device, differing from free expansion in that the pressurized gas expands against a rotatable member, i.e. the turbine rotor. Expansion turbines are capable of doing work, i.e. of converting the expansion of the gas into useful work in the sense that potential energy represented by the gas pressure is transformed into kinetic energy of the motion of the turbine rotor which can be connected to a load for doing work which can be utilized elsewhere in the plant.
In this sense the "work expansion" of a gas is capable of transforming the otherwise useless or wasted energy of a gas at elevated pressure into useful energy and at the same time bring about the expansion which is desired for cooling purposes.
The principle of generating cold by the work expansion of a gas has, of course, long been known and is used in a variety of air and gas rectification systems, i.e. systems resulting in the low-temperature separation of air and other gases into components by, for example, condensation at least in part.
As a consequence, practically all high-capacity gas rectification systems of this type utilize expansion turbines, generally in the form of radial turbines, which are driven by the passage of the expanding gas from the exterior toward the interior.
The work which is generated by the expanded gas is generally recovered by coupling the turbine rotor to an electric-current generator through a transmission, the electrical power of the generator being applied to the plant's electrical network or being supplied to another current-distribution network in proximity to the plant.
However, in conjunction with the generation of electrical energy or as an alternative thereto, it is also possible to dissipate the kinetic energy of the rotor or to produce useful work by connecting the rotor shaft to a fluid-circulator pump, a torque converter or hydrodynamic brake, an air brake or, as is most commonly the case, to a compressor for generating a gas pressure useful elsewhere in the system.
In the latter case, the free end of the turbine shaft is connected to a turbocompressor rotor.
In order to ensure thermal insulation of the expansion-turbine rotor from the turbocompressor rotor the two rotors are generally provided on opposite unsupported ends of a common shaft, within respective housings which are separated by the bearing and seal system which rotatably supports the shaft (whose free ends are cantilevered on the bearings) and any seals between the two housings and the exterior as may be required.
This construction frequently provides a tachometer between the two housing to measure the speed of the shaft and is an extremely expensive structure having the disadvantage that it cannot be operated efficiently in all cases. For example, the turbine and compressor outputs must be precisely dimensioned with respect to one another under all operating conditions of the system.
Such expansion turbine-compressor sets have been used heretofore also in the cooling of natural gases in dewpoint installations or in other systems for the separation of gas-mixture components, e.g. heavier hydrocarbons from the methane of natural gas or the outflow of refineries, chemical plants, metallurgical plants and the like.
Using the principles described above, the natural gas at source pressure is expanded in the expansion turbine and the component consisting predominantly of methane, is compressed for introduction into the pipeline. While the latter component, referred to as a "lean" component because of its reduced concentration of heavier hydrocarbons, consists predominantly of methane, it may nevertheless contain other components and hence it may be more accurate to speak of it as a "lean natural gas."
With reduced natural gas source pressure, the pressure differential between source pressure and consumer pressure (pressure at the discharge side of the compressor) may become so small that the temperature drop which must be brought about by the expansion may be insufficient to effect the dewpoint separation of the high molecular weight components. In such cases, it is known to make use of a refrigerating system, e.g. a propane refrigerant installation to effect exterior cooling of the gases before or upon expansion or even after expansion, or to use systems which increase the pressure differential, e.g. by precompressing the natural gas before expansion. Obviously all of these systems are uneconomical from an energy viewpoint and render the process more complex and expensive.